1. Field of the Invention
The invention relates to defoamer formulations comprising (A) siloxane-based antifoams and (B) polysiloxane copolymers, and also to their application to the defoaming of aqueous media, particularly aqueous media obtained in pulp production.
2. Description of the Related Art
In many liquid systems, especially aqueous systems, which include surface-active compounds purposefully or as unwanted constituents, it is possible for problems to occur as a result of foaming if these systems are contacted more or less intensively with gaseous substances, such as during the gassing of waste waters, during the intensive stirring of liquids, during distillation, washing or coloring operations or during dispensing processes, for example.
This foam can be controlled by mechanical means or through the addition of defoamers. Siloxane-based defoamers have proven particularly appropriate here.
Siloxane-based defoamers are prepared in accordance with DD-A 056 762, for example, by heating hydrophilic silica in polydimethylsiloxanes. This process is very costly and inconvenient, and in spite of this, the activity of the defoamers thus prepared is unsatisfactory. The dispersion of hydrophobized silica in a polydimethylsiloxane, in accordance for example with DE A 29 25 722, is a more rational process, but again the activity of the resulting defoamers is capable of improvement.
Likewise known is the use of modified polyorganosiloxanes in defoamer preparations. Thus, for example, the use of branched siloxane defoamers in combination with polyether-polysiloxane copolymers is recommended as defoamers, for example, for pulp production (EP-A 341 952). In combination with mineral oils as a carrier oil, as well, polyether-polysiloxane copolymers are said to have a positive effect (U.S. Pat. No. 5,523,019). According to WO 98/000216, siloxanes having dimethyl-3-hydroxypropylpolyoxyethylenepolyoxy-propylene groups are particularly suitable surfactants in defoamer formulations.
EP-A 633 225 and EP-A 1076073 claim crosslinked or branched polyorganosiloxanes, respectively, which carry at least one polyether moiety, as one of the two components of a defoamer formulation. The crosslinking in these cases is via alkylene groups, via poly-dimethylsiloxanes or via polyether groups. As a result of the Si—C linkage, the products are stable to hydrolysis.
In contrast to this, the process proposed in U.S. Pat. No. 5,625,024 also leads to Si—O—C-group linkages, which particularly in acidic or basic media are not stable to hydrolysis and therefore lose their activity in foaming aqueous medium.
DE 102 55 649 Å describes defoamer formulations in which specially branched polyether-polysiloxane copolymers are added to known antifoam compositions. In the presence of water, however, defoamer formulations of this kind become less active.
The defoamer formulations prepared in accordance with the prior art either do not always have adequate activity or are not stable to hydrolysis.